It is known to arrange a mobile computing device, such as a personal digital assistant (PDA), so that it can display information relevant to its current environment. For example the article Websigns: Hyperlinking Physical Locations to the Web: Pradhan et al, IEEE Computer Magazine, August 2001, pp. 42–48 discloses a system which links physical entities in the environment to information and resources available on the web. This is done by a system of virtual tags and beacons which are essentially recorded as map references (for tags) and map references and compass bearings (for beacons). When a mobile device is pointed at a position in the real world corresponding to one of these virtual tags or beacons, the pointed-at position is determined by a GPS locating system and digital compass in the device. The device offers the user a link to a web site containing information, or providing services, relevant to that position.